An image forming apparatus such as copying machine, printer, facsimile, printing machine, and multi-functional apparatus may produce an image by transferring a visible image (e.g., toner image) from an image carrier to a recording sheet.
Such a visible image (e.g., toner image) may be fixed on a recording sheet by applying heat and/or pressure to the recording sheet when the recording sheet passes through a fixing unit.
Such a fixing unit may employ a heat roller type or a belt fixing type as heat applying method, for example.
The heat roller type may include a heating roller having a heat source (e.g., halogen lamp) and a pressure roller contactable to the heating roller, wherein heating roller and the pressure roller may form a fixing nip therebetween.
The belt fixing type may include a belt as heat applying member, wherein the belt may have a heat capacitance smaller than a roller.
Further, a fixing unit may employ an electromagnetic induction heating method as heat applying method.
In an electromagnetic induction heating method, a heating roller may include an induction coil therein. When an electric current is applied to the induction coil, an eddy current may be induced in the heating roller when subject to magnetic field generated by the induction coil, by which the heating roller may be heated.
Such a configuration may not need a preheating process for the heating roller, which may be conducted for conventional heat roller type. Accordingly, such an electromagnetic induction heating method may increase a temperature of the heating roller to a given temperature instantaneously.
The induction coil may be applied with a high frequency voltage by a high frequency power source, and the heat roller may include a heat generating layer having magnetic property. The heat generating layer may be heated to a fixing temperature, set approximately to a Curie temperature of the magnetic material used for the heat generating layer, for example.
In such a configuration, when the induction coil is applied with a high frequency voltage by the high frequency power source, the heat generating layer may generate heat.
In such a configuration, a ferromagnetic material contained in the heat generating layer may be heated and generate a magnetic field by the induction coil, and a temperature of the heat generating layer may be instantaneously increased until the ferromagnetic material may be heated to the Curie temperature.
When the temperature of the ferromagnetic material may become the Curie temperature, the ferromagnetic material may lose its magnetic property. When ferromagnetic material loses its magnetic property, the temperature of the ferromagnetic material may not be increased, but may be maintained at a given temperature level.
As above-mentioned, a fixing temperature of the heat generating layer having the ferromagnetic material may be set in a range corresponding the Curie temperature. Accordingly, the fixing temperature of the ferromagnetic material may be maintained at the temperature, which may approximately correspond to the Curie temperature.
Such a fixing unit may have a relatively higher surface releasing-ability and heat resistance of a heat roller.
Furthermore, such a fixing unit may not need a complex control unit, and may shorten a start-up time of heat roller and may control a temperature of a heat roller with a relatively higher precision.
Such a heat roller may be configured with a core metal and resin material layer. When making such a heat roller, a core metal having different shapes and resin material layer having different thicknesses may be used depending on a design concept of a fixing unit. Accordingly, such a heat roller may have a heat capacitance, which may be different from other heat roller having different core metal and resin material layer.
In such a heat roller, a content ratio of ferromagnetic material in a heating layer may be set to a value to adjust or control a start-up time and temperature at a given level.
Further, because the ferromagnetic material may lose its magnetic property at the Curie temperature, toners having magnetic particles may not be attracted to the heat roller with magnetic force of ferromagnetic material at such timing, by which a offset phenomenon or the like may not occur.
Such a fixing unit may be further improved in overheat prevention, and separatability of recording medium, for example.